1. Field of the Invention
The present invention relates to etching methods and etching apparatuses as well as manufacturing methods of semiconductor devices. More particularly, the invention relates to a method of etching a silicon nitride film and an etching apparatus for performing the etching method, as well as a method of manufacturing a semiconductor device employing that etching method.
2. Description of the Background Art
Semiconductor devices such as a DRAM (Dynamic Random Access Memory) have been known. These semiconductor devices are becoming finer and integration thereof is progressing to a greater degree. Accordingly, there arises a need for finer processing dimensions in a manufacturing process of the semiconductor devices. Further, a mask alignment margin is being reduced that is provided in consideration of mask alignment errors in a photolithography process step of the manufacturing process of the semiconductor devices.
An approach employed for addressing such finer processing dimensions is to form a self-aligned contact as described below, in a step of forming a contact hole between gate electrodes of a plurality of field effect transistors.
Specifically, according to the approach to form the self-aligned contact, sidewalls of the gate electrodes are covered with an insulating film to form a sidewall film. As a material for the insulating film constituting the sidewall film, a silicon nitride film or the like is used having a higher selective etching ratio with respect to a silicon oxide film which is a material for an interlayer insulating film. After the sidewall film is formed, the interlayer insulating film formed of the silicon oxide film is deposited on the gate electrodes. In this interlayer insulating film, a contact hole is formed between the gate electrodes. In an etching step for producing the contact hole, assume that the position of the contact hole is shifted from a predetermined position to a certain extent. As the sidewall film made of the silicon nitride film is formed on the sidewalls of the gate electrodes, this etching step never causes the contact hole to reach the gate electrodes. Even if the contact hole is filled with a conductive film, it is possible to prevent the conductive film and the gate electrodes from being short-circuited.
As shown in FIGS. 14-17, in order to produce such a self-aligned contact, it is necessary to form the silicon nitride film to cover the gate electrodes and thereafter partially remove the silicon nitride film by anisotropic etching thereby form a sidewall film formed of the silicon nitride film. FIGS. 14-17 are cross sections showing a conventional method of manufacturing a semiconductor device, illustrating an etching step for forming the sidewall film. The conventional method of manufacturing a semiconductor device is now described in conjunction with FIGS. 14-17.
Referring to FIG. 14, a silicon oxide film 111 constituting a gate insulating film is deposited on a substrate 110 such as a silicon substrate. Silicon oxide film 111 here is 3 nm in thickness. On silicon oxide film 111, a two-layer film 112 made of a tungsten silicide film and a polysilicon film is formed to produce gate electrodes. The tungsten silicide film and the polysilicon film each have a thickness of 50 nm. A silicon nitride film 113 is formed on two-layer film 112. Silicon nitride film 113 is 150 nm in thickness. A resist film (not shown) is formed on silicon nitride film 113. This resist film is photolithographically processed to form a resist film 114 having a gate pattern.
Resist film 114 is used as a mask to partially remove, through etching, silicon nitride film 113 thereby produce upper silicon nitride films 120a and 120b. Resist film 114 is thereafter removed. Upper silicon nitride films 120a and 120b are used as masks to partially remove, through etching, two-layer film 112 thereby produce gate electrodes 115a and 115b. The structure as shown in FIG. 15 is thus achieved.
Referring to FIG. 16, a silicon nitride film 116 is formed to cover gate electrodes 115a and 115b. Silicon nitride film 116 is 40 nm in thickness.
Silicon nitride film 116 is partially removed through anisotropic etching to form sidewall films 117a-117d formed of the silicon nitride film on the sidewalls of gate electrodes 115a and 115b as shown in FIG. 17.
In the etching process for forming sidewall films 117a-117d, if silicon nitride film 116 does not have its selective ratio higher enough relative to silicon oxide film 111, silicon oxide film 111 is partially eliminated by this etching as shown in FIG. 17. Consequently, there are generated a portion of the substrate cut away through this etching (substrate cutaway portion) 144 and a damaged portion 145 having defects in substrate 110. This damage of substrate 110 causes a deteriorated reliability of a semiconductor device including components such as field effect transistors formed on substrate 110.
Various techniques have accordingly been proposed for preventing such substrate cutaway portion 144 and damaged portion 145 of substrate 110.
According to a method disclosed in Japanese Patent Laying-Open No. 11 -145113 for example, a silicon nitride film is etched to form a sidewall film by using a parallel plane RIE (Reactive Ion Etching) apparatus and chlorine gas as a reactant gas. The frequency of a high-frequency power supply and the power supplied to upper and lower electrodes are optimized to increase the selective ratio of the silicon nitride film with respect to a silicon oxide film (i.e. etching rate of silicon nitride film/etching rate of silicon oxide film).
Nevertheless, the selective ratio of the silicon nitride film relative to the silicon oxide film disclosed in Japanese Patent Laying-Open No. 11-145113 is approximately 10, and this value of the selective ratio is insufficient. Therefore, damages to silicon oxide film 111 and substrate 110 as shown in FIG. 17 cannot surely be avoided.
According to a technique proposed in Japanese Patent Laying-Open No. 10-50660, a silicon nitride film is etched to form a sidewall film by using as a reactant gas a mixed gas of chlorine gas and hydrogen bromide gas and consequently the selective ratio of the silicon nitride film relative to a silicon oxide film is approximately 20.
An inventor reviewed this technique and found that an increased ratio of the hydrogen bromide in the reactant gas as described above would result in a problem as shown in FIG. 18 that deposition on sidewall films 117a-117d occurs in the etching process, and the deposition parts have functions as a mask in etching to cause partially abnormal shapes of sidewall films 117a-117d. FIG. 18 is a cross section illustrating the abnormal shapes of sidewall films 117a-117d. Such abnormality in the shape of sidewall films 117a-117d (shape defect) is one factor of any step on the upper surface of an interlayer insulating layer for example formed on gate electrodes 115a and 115b. Such a step may cause failures like disconnection in an interconnection layer formed on the interlayer insulating layer. As a result, the reliability of devices formed on substrate 110 deteriorates.
One object of the present invention is to provide an etching method that enables the selective ratio of a silicon nitride film to be sufficiently higher relative to a silicon oxide film without causing shape defects of the etched silicon nitride film, and an etching apparatus for applying this etching method.
Another object of the invention is to provide a method of manufacturing a semiconductor device having a high reliability, the semiconductor device being formed by using an etching method enabling the selective ratio of a silicon nitride film to be sufficiently higher relative to a silicon oxide film without causing shape defects of the etched silicon nitride film.
According one aspect of the invention, an etching method for selectively etching a silicon nitride film formed on a substrate on which a silicon oxide film and the silicon nitride film are formed uses a reactive ion etching apparatus including a process chamber holding therein the substrate to be etched, gas supply means for supplying a reactant gas into the process chamber, and an upper electrode and a lower electrode provided in the process chamber and applied with respective high-frequency currents. The substrate is placed on the lower electrode. The etching is performed under the conditions that the reactant gas includes halogen-based gas, the reactant gas in the process chamber has a pressure of at least 12.0 Pa (90 mTorr) and at most 66.7 Pa (500 mTorr), the reactant gas is supplied into the process chamber at a flow rate of at least 0.1 liter/min (100 sccm) and at most 0.55 liter/min (550 sccm), and the distance between the upper electrode and the lower electrode is at least 50 mm and at most 120 mm.
The reactive ion etching apparatus having the upper and lower electrodes can thus be used to etch the silicon nitride film with a sufficiently high selective ratio of the silicon nitride film relative to the silicon oxide film. For the substrate with the silicon oxide film and the silicon nitride film formed thereon, it is possible to prevent the silicon oxide film from being partially removed when the silicon nitride film is selectively etched. In this way, occurrence of any defect such as damage to the substrate can be avoided that is due to removal of the silicon oxide film. It is further possible to avoid defect in the shape of the silicon nitride film from occurring in the etching process, according to the present invention.
The reasons for setting the pressure of the reactant gas at 12.0 Pa-66.7 Pa are as follows. When the pressure is lower than 12.0 Pa, a sufficiently high selective ratio of the silicon nitride film relative to the silicon oxide film is difficult to achieve. On the other hand, when the pressure exceeds 66.7 Pa, unstable discharging occurs that is caused by application of high-frequency currents to the upper and lower electrodes in the process chamber, resulting in unstable etching. Further, in the etching process, a deposition process dominates an etching process which extremely lowers the selective ratio. Otherwise, the selective ratio of the substrate surface being etched varies to a greater extent (i.e., the uniformity of the selective ratio deteriorates).
The reasons for setting the flow rate of the reactant gas at 0.1 liter/min (100 sccm)-0.55 liter/min (550 sccm) are as follows. When the flow rate is lower than 0.1 liter/min, the selective ratio of the substrate surface being etched varies to a greater extent (the uniformity of the selective ratio deteriorates). On the other hand, when the flow rate exceeds 0.55 liter/min, the reactant gas in the process chamber is difficult to be discharged by a pump. Here, 1 sccm (standard cubic centimeter per minute) means that the flow rate of gas at 0xc2x0 C. and 1 standard atmosphere is 1 cm3 per minute.
The distance between the upper and lower electrodes (electrode interval) is defined as 50 mm-120 mm for the reason that the electrode interval smaller than 50 mm or greater than 120 mm would deteriorate the uniformity of the selective ratio of the silicon nitride film with respect to the silicon oxide film.
Preferably, the etching method according to the one aspect of the invention is performed under the conditions that the high-frequency current supplied to the upper electrode has a frequency of at least 12 MHz and at most 60 MHz and the high-frequency current supplied to the lower electrode has a frequency of 13.56 MHz.
A high-frequency supply for generating high-frequency current in such a frequency range is easily available in general, and the manufacturing cost of the etching apparatus, to which the etching method of the invention is applied, can be prevented from rising.
Preferably, the etching method according to the one aspect of the invention may be performed under the conditions that the high-frequency current applied to the upper electrode has a frequency of 13.56 MHz and respective high-frequency currents supplied to the upper and lower electrodes are different in the phase (out of phase) from each other.
In this case, the high-frequency currents can be supplied to the upper and lower electrodes from a high-frequency supply at 13.56 MHz, which is especially in common use.
According to another aspect of the invention, an etching method for selectively etching a silicon nitride film formed on a substrate on which a silicon oxide film and the silicon nitride film are formed uses an inductively coupled plasma etching apparatus including a process chamber holding therein the substrate to be etched, a coil for generating plasma for etching in the process chamber, and gas supply means for supplying a reactant gas into the process chamber. The etching is performed under the conditions that the reactant gas includes halogen-based gas, the distance between the substrate and the coil is at least 80 mm and at most 1000 mm, the reactant gas in the process chamber has a pressure of at least 0.5 Pa (3.7 mTorr) and at most 66.7 Pa (500 mTorr), and the reactant gas is supplied into the process chamber at a flow rate of at least 0.05 liter/min (50 sccm) and at most 0.55 liter/min (550 sccm).
The inductively coupled plasma etching apparatus can thus be used to etch the silicon nitride film with a sufficiently high selective ratio of the silicon nitride film relative to the silicon oxide film. For the substrate with the silicon oxide film and the silicon nitride film formed thereon, it is possible to prevent the silicon oxide film from being partially removed when the silicon nitride film is selectively etched. In this way, occurrence of any defect such as damage to the substrate can be avoided that is due to removal of the silicon oxide film. It is further possible to avoid defect in the shape of the silicon nitride film from occurring in the etching process, according to the present invention.
The distance between the substrate and the coil that is smaller than 80 mm deteriorates the uniformity in the substrate surface of the etching selective ratio of the silicon nitride film relative to the silicon oxide film. The distance between the substrate and the coil that exceeds 1000 mm lowers the etching rate of the silicon nitride film, making it difficult to smoothly etch the silicon nitride film.
The pressure of the reactant gas is set at 0.5 Pa-66.7 Pa for the following reasons. If the pressure is lower than 0.5 Pa, a sufficiently high selective ratio of the silicon nitride film relative to the silicon oxide film is difficult to obtain. On the other hand, if the pressure is higher than 66.7 Pa, the plasma generated in the process chamber is unstable which deteriorates stability in etching. Further, in the etching process, a deposition process dominates an etching process and thus the selective ratio becomes extremely lower or the uniformity of the selective ratio of the substrate surface being etched declines. In the inductively coupled plasma etching apparatus, which is compared here with the reactive ion etching apparatus including the upper and lower electrodes, the reactant gas can have the pressure with its lower limit smaller than that in the reactive ion etching apparatus, the reasons for this are as follows. The inductively coupled plasma etching apparatus uses the amount of the high-frequency current supplied to the coil to control the amount of generated plasma or etchants (ions or radical for etching) while sets the lower electrode provided under the substrate at a predetermined potential (i.e. a bias current is supplied to the lower electrode) to attract the etchants toward the substrate. In other words, the amount of generated etchants and attraction of the etchants toward the substrate can separately be controlled. Therefore, even if the reactant gas has a low pressure, a satisfactory selective ratio is achieved by setting the lower electrode at a predetermined potential (i.e. the bias is applied).
The flow rate of the reactant gas is defined as 0.05 liter/min-0.55 liter/min for the reasons below. If the flow rate is less than 0.05 liter/min, the uniformity of the selective ratio of the substrate surface being etched deteriorates. The flow rate exceeding 0.55 liter/min makes it difficult to discharge the reactant gas within the process chamber by a pump.
According to still another aspect of the invention, an etching method for selectively etching a silicon nitride film formed on a substrate on which a silicon oxide film and the silicon nitride film are formed uses an etching apparatus including a process chamber holding therein the substrate to be etched, gas supply means for supplying a reactant gas into the process chamber, an antenna for supplying into the process chamber a high-frequency wave in the ultra high frequency range, and a coil for generating a magnetic field in the process chamber. The etching is performed under the conditions that the reactant gas includes halogen-based gas, the reactant gas in the process chamber has a pressure of at least 0.5 Pa (3.7 mTorr) and at most 66.7 Pa (500 mTorr), and the reactant gas is supplied into the process chamber at a flow rate of at least 0.05 liter/min (50 sccm) and at most 0.55 liter/min (550 sccm).
The etching apparatus under these conditions can be used to etch the silicon nitride film by plasma produced by the high-frequency wave supplied by the antenna into the process chamber and the magnetic field such that the selective ratio of the silicon nitride film relative to the silicon oxide film is sufficiently high. For the substrate with the silicon oxide film and the silicon nitride film formed thereon, it is possible to prevent the silicon oxide film from being partially removed when the silicon nitride film is selectively etched. In this way, occurrence of any defect such as damage to the substrate can be avoided that is due to removal of the silicon oxide film. It is further possible to avoid defect in the shape of the silicon nitride film from occurring in the etching process, according to the present invention.
The reasons for setting the pressure of the reactant gas at 0.5 Pa-66.7 Pa are as follows. When the pressure is lower than 0.5 Pa, a sufficiently high selective ratio of the silicon nitride film relative to the silicon oxide film is difficult to achieve. On the other hand, when the pressure exceeds 66.7 Pa, the plasma generated in the process chamber is unstable, resulting in instability of etching.
The reasons for setting the flow rate of the reactant gas at 0.05 liter/min-0.55 liter/min are as follows. When the flow rate is lower than 0.05 liter/min, the uniformity of the selective ratio of the substrate surface being etched is impaired. On the other hand, when the flow rate exceeds 0.55 liter/min, the reactant gas in the process chamber is difficult to be discharged by a pump.
Preferably, the etching method according to the one, another, or still another aspect of the invention uses the reactant gas including at least one selected from the group consisting of chlorine gas and hydrogen bromide gas.
Accordingly, the etching rate of the silicon oxide film can be kept low and thus the selective ratio of the silicon nitride film relative to the silicon oxide film can be increased.
Preferably, the etching method according to the one, another, or still another aspect of the invention uses the reactant gas including at least one selected from the group consisting of chlorine gas (Cl2), a mixed gas of chlorine gas and oxygen gas (Cl2/O2), a mixed gas of chlorine gas and hydrogen bromide gas (Cl2/HBr), a mixed gas of hydrogen bromide gas and oxygen gas (HBr/O2), a mixed gas of chlorine gas, hydrogen bromide gas and oxygen gas (Cl2/HBr/O2), a mixed gas of hydrogen bromide gas and sulfur hexafluoride gas (HBr/SF6), a mixed gas of chlorine gas and carbon tetrafluoride gas (Cl2/CF4), a mixed gas of hydrogen bromide gas and carbon tetrafluoride gas (HBr/CF4), and a mixed gas of chlorine gas and sulfur hexafluoride gas (Cl2/SF6).
Then, the silicon nitride film can be etched maintaining a sufficiently enhanced selective ratio of the silicon nitride film relative to the silicon oxide film.
Preferably, by the etching method according to the one, another or still another aspect of the invention, etching is performed in the process chamber in which provided a member having a surface layer containing aluminum.
In this case, ions and the like as etchants enter the surface layer of the member in the etching process and consequently the etching atmosphere contains aluminum discharged from the surface layer. The inventor has found that the selective ratio of the silicon nitride film relative to the silicon oxide film dramatically improves in such an atmosphere. Then, the silicon nitride film can be etched with a higher selective ratio thereof with respect to the silicon oxide film.
The reason for this is considered as follows. The presence of aluminum in the etching atmosphere allows an aluminum compound (supposedly aluminum-silicon-oxygen compound) to be deposited selectively on the silicon oxide film. This aluminum compound functions as a protective coating for the silicon oxide film. As a result, the selective ratio of the silicon nitride film relative to the silicon oxide film can be improved.
Preferably, by the etching method according to the one, another or still another aspect of the invention, the silicon nitride film is etched by supplying into the process chamber gas containing aluminum.
The etching atmosphere thus contains aluminum. Accordingly, an aluminum compound is selectively deposited on the silicon oxide film and the silicon nitride film can be etched maintaining an enhanced selective ratio of the silicon nitride film relative to the silicon oxide film.
According to a further aspect of the invention, an etching method for selectively etching a silicon nitride film formed on a substrate on which a silicon oxide film and the silicon nitride film are formed uses an etching apparatus including a process chamber holding therein the substrate to be etched and in which plasma for etching is produced. The silicon nitride film is etched in the process chamber provided with a member therein having a surface layer including aluminum.
In the etching process under these conditions, ions and the like as etchants enter the surface layer of the member and consequently the etching atmosphere contains aluminum discharged from the surface layer. An aluminum compound is then deposited selectively on the silicon oxide film so that the silicon nitride film can be etched with a higher selective ratio thereof with respect to the silicon oxide film.
Preferably, according to the etching method according of the further aspect of the invention, the member is grounded.
Accordingly, it is ensured that ions and the like enter the surface layer of the member and aluminum is supplied into the etching atmosphere. The aluminum compound is thus sure to be formed on the silicon oxide film and the silicon nitride film can be etched with the selective ratio thereof relative to the silicon oxide film that is surely improved.
According to a still further aspect of the invention, an etching method for selectively etching a silicon nitride film formed on a substrate on which a silicon oxide film and the silicon nitride film are formed uses an etching apparatus including a process chamber holding therein the substrate to be etched for dry-etching the silicon nitride film. The silicon nitride film is etched by supplying into the process chamber gas containing aluminum.
Accordingly, the etching atmosphere includes aluminum and an aluminum compound is selectively deposited on the silicon oxide film. The silicon nitride film can thus be etched with an increased selective ratio thereof relative to the silicon oxide film.
Preferably, by the etching method according to the one, another, still another or still further aspect of the invention, the gas containing aluminum is organoaluminum-based gas.
When organoaluminum in liquid state is employed, inert gas as carrier gas is blown into the liquid organoaluminum (bubbling), and the bubbled inert gas is supplied to the process chamber. Then, organoaluminum-based gas can relatively easily be supplied to the process chamber for etching.
According to a still further aspect of the invention, an etching apparatus is used to apply thereto the etching method according to one, another, still another, further, or still further aspect of the invention.
This etching apparatus can be used to easily etch the silicon nitride film maintaining an enhanced selective ratio of the silicon nitride film relative to the silicon oxide film.
According to a still further aspect of the invention, a semiconductor device is manufactured by using the etching method according to one, another, still another, further, or still further aspect of the invention.
It is thus possible, in a process of etching a silicon nitride film on a substrate on which a silicon oxide film and the silicon nitride film are formed, to prevent damage to the silicon oxide film. Then, any defect or failure of the semiconductor device due to such damage can be avoided.
According to a still further aspect of the invention, a method of manufacturing a semiconductor device uses the etching method according to one, another, still another, further, or still further aspect of the invention.
In a manufacturing process of the semiconductor device, it is possible to avoid, in an etching step of a silicon nitride film on a substrate on which a silicon oxide film and the silicon nitride film are formed, any damage to the silicon oxide film. Then, any defect or failure of the semiconductor device due to such damage can be avoided.